Asphalt pavement macadam spreading technology is widely used in engineering scenes such as synchronous macadam seal layer, interlayer bonding seal layer, bridge deck waterproof seal layer, permeable layer cover, preventive maintenance of old road, etc. It is a key process to improve pavement interlayer bonding force, waterproof impermeability, anti-skid wear resistance and interlayer shift resistance.The thickness, particle size, uniformity and coverage rate of crushed stone spreading directly determine the construction quality of sealing layer. Too thin spreading will cause oil exposure, insufficient bond strength and waterproof failure, resulting in void, slip and cracking between pavement layers; too thick spreading will cause crushed stone accumulation, loose rolling and poor flatness, increasing pavement rutting risk and material loss.According to the Technical Specification for Highway Asphalt Pavement Construction (JTG F40-2004) and the special construction standards of highway pavements in various provinces, this paper combs the thickness, dosage, particle size matching and construction control points of various asphalt pavement macadam spreading in detail, and provides standardized basis for field construction and quality inspection and acceptance.

Core design principle of crushed stone spreading thickness

Asphalt pavement macadam spreading is not simply uniform paving thickness, it needs to follow the four core principles of particle size matching, thin thickness adaptation, coverage controllable and interlayer coordination to meet the functional requirements of different construction scenarios.Firstly, the principle of matching particle size and thickness, the spreading compaction thickness of crushed stone shall be matched with the nominal maximum particle size of aggregate, and the conventional spreading thickness shall be slightly larger than the particle size of crushed stone to ensure the stable embedding and extrusion of aggregate; secondly, the principle of functional adaptation, the waterproof bonding type seal layer is thin, and the anti-skid maintenance type seal layer is moderately thickened; thirdly, the principle of controllable coverage, mainly uniform coverage, to prevent full paving and accumulation, and to avoid interlayer isolation; Fourthly, the principle of working condition adaptation: the thickness and dosage of expressway main line, ramp, bridge deck and ordinary highway shall be controlled differentially to adapt to different load conditions.

Standards for thickness and dosage of crushed stone spreading in different construction scenarios

Asphalt pavement macadam spreading is mainly divided into four mainstream processes: synchronous macadam sealing layer, transparent macadam spreading layer, bridge deck macadam sealing layer and fine macadam surface. There are clear specification limits for various process thickness, particle size and spreading amount, which are the core standards for site construction.

Synchronous crushed stone seal layer (high-speed main line, maintenance and transformation core process)

Synchronous crushed stone sealing is the core technology of waterproof and bonding between asphalt pavement layers of expressway. The synchronous construction mode of asphalt spraying and crushed stone spreading is adopted to adapt to the maintenance of old and new pavement and the sealing construction between surface layers.According to the highway pavement quality control standard of the Department of Transportation, the standard spreading thickness of synchronous gravel seal layer shall be controlled at 6~12mm, and the optimal thickness under normal working conditions shall be 8~10 mm.Single particle size clean crushed stone shall be selected, with common specifications of 5~10mm and 10~15mm, and corresponding spreading dosage of 8~12kg/㎡.The coverage rate of construction core shall be controlled to evenly cover 60%~70% of the pavement, which not only ensures the adhesion and waterproof effect between asphalt layers, but also avoids the interlayer isolation caused by full gravel paving, and eliminates the later pavement movement and peeling diseases.Heavy load, long downhill, ramp and other special sections can be moderately thickened, the maximum spreading thickness shall not exceed 12mm, and the upper limit of dosage shall be 12kg/㎡.

Penetrating layer stone chip spreading (base layer and asphalt surface layer are connected)

After the cement stabilized macadam base is sprayed with prime asphalt, fine stone chips shall be spread timely to protect the prime layer and seal the pores of the base. This scenario is mainly thin-layer spreading without heavy structure requirements.It is specified that the amount of stone chips shall be 2~3m³/1000㎡, and the spreading thickness after compaction shall be controlled within 3~5 mm.Clean stone chips with a particle size of ≤5mm are selected and spread thinly and evenly without exposing black oil, covering the surface of the prime coat asphalt completely. The main function is to prevent the prime coat asphalt from bonding to construction vehicles and protecting the grassroots health interface. There is no need for thick paving. It is strictly prohibited to accumulate sand.

Asphalt macadam seal layer of bridge deck

The gravel sealing layer under the bridge asphalt pavement shall give consideration to waterproof, bonding and shock absorption functions, and the thickness control shall be more precise to avoid excessive thickness resulting in large overall weight of bridge deck pavement and excessive thickness resulting in water seepage and void.5~ 10mm single-grain macadam shall be adopted for the conventional bridge deck macadam sealing layer, the standard spreading thickness shall be 6~ 8mm, the spreading amount shall be controlled at about 10kg/㎡, and the modified asphalt spreading amount shall be matched with 1.6~2.0kg/㎡, so as to ensure that the macadam and asphalt are fully bonded and adapt to the deformation characteristics of bridge structure.

Fine crushed stone surface (preventive maintenance thin layer process)

Fine crushed stone surface is mostly used for preventive maintenance of ordinary highway and high-speed light-load section, which is divided into single-layer and double-layer technology.The compacted thickness of single-layer fine crushed stone surface is 4~6mm, and the spreading amount is 100%~110% of the full paving standard amount; the spreading thickness of the lower layer of double-layer technology is about 4mm, and the dosage is 80%~90% of the full paving amount; the thickness of the upper layer is 4~5mm, and the dosage is 95%~105% of the full paving amount. The total thickness of the whole double-layer is controlled at 8~10mm, giving consideration to anti-skid, wear resistance and water-sealing curing effects.

Hard requirements for matching spreading thickness and crushed stone particle size

In order to ensure that the crushed stone is compacted and firmly bonded, and to prevent loose falling, the construction must strictly follow the grain size and thickness matching specification, and the mismatch construction is strictly prohibited:
- Grain size 4.75~9.5mm crushed stone: suitable for spreading thickness 5~ 8mm, mostly used for transparent layer armor and light sealing layer;
- Gravel size 10~15mm: suitable for spreading thickness 8~12mm, mostly used for high-speed main line synchronous gravel seal layer, heavy load section maintenance;
- Thickness limit requirements: the spreading thickness of crushed stone shall not be less than the maximum particle size of aggregate to avoid hanging crushed stone and weak bonding; the maximum thickness shall not exceed 2 times of particle size to prevent stacking and compaction of crushed stone overhead.

Construction thickness deviation and acceptance standard

Asphalt pavement macadam spreading belongs to functional thin layer construction, and acceptance takes forming thickness, spreading uniformity, coverage rate and bonding quality as core indexes, which is different from asphalt pavement thickness acceptance standard.
Thickness deviation control: the allowable thickness deviation of spreading molding under normal working conditions is ±2mm, and there is no large area of uneven thickness, local accumulation and local oil exposure.Coverage acceptance: The coverage rate of the highway main line and bridge deck seal is stable at 60%~70%, and the coverage rate of the maintenance surface is ≥90%.Appearance acceptance: the crushed stones are evenly distributed, free from white, accumulation and threshing, and are embedded firmly after rolling, free from wheel sticking and loose sand.The inspection methods are mainly on-site appearance inspection, thickness gauge measurement and point sampling verification, and the qualified rate shall reach more than 95%.
Five, thickness uniformity construction quality control points
Uneven thickness and partial exceeding of standard of crushed stone spreading are high-frequency problems in construction, which directly affect the quality of sealing layer. Full-process control shall be implemented on site:
First, equipment calibration uniform operation.Calibrate the discharge amount and paving width of the crushed stone spreader before construction, keep the uniform speed during construction, control the operation speed at 3~6km/h, and prevent local thick paving and missing paving caused by rapid acceleration and sudden braking.
Second, strictly control the quality of raw materials.Select dry, clean, impurity-free and weather-free single-particle crushed stone, with mud content and dust content up to standard, so as to avoid binding failure and uneven thickness caused by dust wrapping aggregate.
Third, man-machine coordination fine leveling.After mechanical spreading, arrange manual follow-up leveling to sweep away accumulated gravel in time and fill in areas where oil is leaked to ensure uniform overall thickness.
Fourth, control the construction schedule.The synchronous crushed stone sealing layer shall be spread immediately after asphalt spreading to ensure that the asphalt temperature meets the bonding requirements; after the construction is completed, the traffic shall be closed in time, and the vehicles shall be prohibited from passing and rolling to prevent the displacement, thickness deformation and local threshing of crushed stones.

Conclusion

The thickness of asphalt pavement macadam spreading is the key index to determine the waterproof, adhesive and anti-skid performance of sealing layer. The differential control of thickness, particle size and dosage in different construction scenarios is the core to ensure the integrity of pavement layers and eliminate early diseases.During the construction, it is necessary to strictly follow the specification standards, accurately match the particle size and thickness, strictly control the spreading dosage and coverage rate, ensure the uniform spreading thickness and firm bonding of crushed stones through the whole process control of equipment calibration, uniform construction, manual finishing and closed maintenance, give full play to the waterproof, shear resistance and surface protection functions of the sealing layer structure, and effectively improve the overall stability and service life of asphalt pavement.